Loss of gene function: Clues from expressed gene fragments in Rickettsia

Many obligate intracellular pathogens have small genomes with high fractions of pseudogenes. A recent analysis of gene expression patterns in Rickettsia conorii shows that short open reading frames inside deteriorating genes are occasionally transcribed into RNA . Here, we show that substitution frequencies at nonsynonymous sites are similar for expressed and unexpressed parts of the fragmented genes. We conclude that the observed expression is a temporary stage in the gene degradation process, suggesting that the expressed gene fragments are not functional.